Measuring and testing – Volume or rate of flow – Expansible chamber
Patent
1988-06-28
1991-02-19
Goldstein, Herbert
Measuring and testing
Volume or rate of flow
Expansible chamber
73249, G01F 318
Patent
active
049932624
DESCRIPTION:
BRIEF SUMMARY
The invention relates to apparatus for monitoring flow of a fluid along a conduit, and to a system including such apparatus.
The conduit may contain water or gas, which is for example part of a domestic water or gas supply.
Present flow meters for water flow measurement do not lend themselves without considerable increasing in manufacturing costs to high accuracy at low flow rates, generally are not susceptible of supporting a remote information logger, and in any event do not readily meet the criteria laid down in BS 5728 (amendment 1-1985) Class D. Domestic water flow meters according to this standard are required to have a nominal flow rate of 1.0 m.sup.3 /hr.
This requires a starting flow rate of 0.00375 m.sup.3 /h, a minimum flow rate of 0.0075 m.sup.3 /h, above which accuracy is to be within +/-5% rate to a transiton flow rate of 0.0115 m.sup.3 /h, above which accuracy is to be within +/-2% rate to a maximum flow rate of 2.0 m.sup.3 /h. Pressure loss accordingly is to be a maximum of 1 bar at the maximum flow rate and a maximum of 0.25 bar at the nominal flow rate.
Domestic water pipework internal diameter is in the United Kingdom nominally 15 mm with a tolerance of +/-1 mm. At minimum and maximum flow rates, mean fluid pipework velocities are therefore 0.012 m/s and 3.14 m/s based on 15 mm dia.
Corresponding Reynolds numbers with water at ambient conditions are 135 and 36,000. The flow in the pipework therefore goes from laminar to turbulent over the flow range.
Turndown: maximum flow rate/minimum flow rate is very high at 267:1.
The present flow meters used in the United Kingdom and elsewhere, are generally rotary piston meters. They usually comprise a cylindrical measurement chamber with a partition plate separating the inlet from the outlet port. The piston is also cylindrical and has numerous holes in its support so that liquid is able to flow on both sides of the piston. The piston is guided within the measuring chamber by the circulation of the piston hub or peg between an outer and inner boss. The slot in the piston for the partition plate confines motion relative to the plate to one plane. The motion of the piston is thus oscillatory.
As the flow meter entraps fixed quantities of liquid outside and inside the piston each revolution, the rate of flow is proportional to rotational velocity.
Operation of this meter over a wide turndown depends on close tolerances to give sufficiently low internal leakage especially as the leakpaths are constrained to be short relative to breadth because of the rounded geometry. Furthermore the tolerance structure is complex to keep mechanical friction low. For example the tolerances on the piston hub, the internal diameter of the outer boss confining its path, the OD of the piston and the ID of the measuring chamber must be such that the piston outer circumference does not touch and hence rub on the inside circumference of the measuring chamber. Otherwise mechanical friction will go up.
This is because the constraining forces are designed to be taken by the hub on the ID of the outer boss, which confines them to a smaller radius arm, so keeping friction torque down. Conversely the piston outer circumference must be close enough to the inner circumference of the measuring chamber to keep internal leakage low.
To achieve Class D Specifications and retain existing maximum flow capability the meter would require even tighter tolerances. Without some form of compensating measure, tightening the clearance between the OD of the piston and the ID of the measuring chamber would reduce life as less wear at the hub would be required to make the circumferences touch and frictional torque increase. In order to control the complex tolerance structure of this meter cost would considerably increase, with no guarantee on accuracy being increased commensurately.
Moreover, it has been estimated that in the Thames Water Authority area alone there could be a significant drop in demand (present day) if there was adoption of universal metering.
It is accordingly an object of the inve
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